<p>The headwater catchment provides primary freshwater sources for local cities. Understanding the seasonal and spatial characteristics of hydrochemistry and the controlling mechanism in surface water in headwater catchments is highly important. We selected a headwater catchment, the Jiangwan watershed, in the Taihu Lake Basin for hydrochemical studies. Statistical analysis, Piper diagrams, Gibbs diagrams, and ion ratio methods were applied to investigate the hydrochemical features of the river and analyze the sources of water ions. The results revealed that the dominant cations and anions in the surface water were Ca²⁺ and HCO₃^– and most surface water bodies were predominantly classified as the HCO₃–Ca·Na type. The ion concentrations generally decreased in spring and increased in winter. Along the main stream, the concentrations of earth alkali ions and alkalinity peaked at the midstream sampling site M2. In general, a downstream increase of ion concentrations was apparent, alongside with human pressures. However, the ion concentrations were merely influenced by seasonal variations. The hydrochemical signature of surface water is principally controlled by rock weathering processes, in which silicate weathering is the dominant source of major ions, with carbonate weathering co-contributing. This study is critical for informing strategies aimed at the integrated management and ecological preservation of a headwater catchment in the Lake Taihu Basin.</p>